Yes of course, but it's inaccurate in the way it measures your effort. Another example is using a turbo trainer. If you use a turbo with your garmin with the GPS turned off but have a speed/cadance pod attached it will record your speed. Holding 20mph on the turbo takes significant effort, but it much easier on the road (flat), yet they still record as the same level of calories. This is one of the reasons why powermeters are used for professional training, it's a direct measurement of effort at any given time.
I thought we were talking about heart rate, not distance/speed.
Anyway, while I appreciate that a power meter will allow you to calculate the energy required to
move the bike, I've argued before (and even quoted the laws of thermodynamics to back up my argument) that it doesn't tell the whole story of what the body is doing, so a power meter can't be used to calculate calories burned accurately either. As far as I understand it, professionals use power meters (in conjunction with HRMs) because they're trying to increase their power output for a given heart rate, not to tell them how much they can eat.
To answer the OP, none of the available methods of calculating calories burned is accurate. The calories to maintain your body weight that whatever website you're using has given you aren't accurate either - they're based on averages. You need to experiment with the different options, and figure out which estimates come closest for you.